This is interesting. A wind powered vehicle that can travel faster than the wind downwind - ie parallel to the wind, in the direction of wind.

It has generated controversy, with a lot of people calling it a hoax. But some prominent scientists have defended the claims. I cannot see any way it breaks the fundamental laws of physics, eg conservation of energy. It is essentially a very complicated gearbox that utilises the relative speed between the wind and the ground.

The explanations given in the video kind of miss the point though.

https://www.youtube.com/watch?v=jyQwgBAaBag

On a side note - I read online that a boat can also travel faster than the wind in the downwind direction, by jibing. But I cannot figure out how. As far as I can tell once the downwind speed (vector component) matches the wind speed, it looses all propulsion from the wind.

Terrific.
About 6'30"

Works sort of how a sail boat tacks when the wind is behind it.
The sail boat moves MUCH faster than the wind.

Sails work by vacuum. The boat is sucked along, it is not blown by the wind.

Sprintcyclist wrote on May 9^th, 2022 at 8:11pm:

Terrific. About 6'30" Works sort of how a sail boat tacks when the wind is behind it. The sail boat moves MUCH faster than the wind. Sails work by vacuum. The boat is sucked along, it is not blown by the wind.

The limiting factor is drag, back in my windsurfing days, once planing the only bit in the water is the tip of the fin. Now with foils on all sorts of craft, drag is even less so they go much faster than the wind.

Sprintcyclist wrote on May 9^th, 2022 at 8:11pm:

Terrific. About 6'30" Works sort of how a sail boat tacks when the wind is behind it. The sail boat moves MUCH faster than the wind. Sails work by vacuum. The boat is sucked along, it is not blown by the wind.

1/2 right. If you travel exactly downwind you can only go as fast as the wind.
At beam or broad reach, you get push from the wind and lift from the apparent wind. The lift from the way the air travels around your sail adds the extra speed that allows your to travel faster than the wind.

freediver wrote on May 9^th, 2022 at 7:59pm:

This is interesting. A wind powered vehicle that can travel faster than the wind downwind - ie parallel to the wind, in the direction of wind. On a side note - I read online that a boat can also travel faster than the wind in the downwind direction, by jibing. But I cannot figure out how. As far as I can tell once the downwind speed (vector component) matches the wind speed, it looses all propulsion from the wind.

Yes there are many sailboats that can sail faster than the wind across the wind around 80 deg to true wind down to about 135 deg to true wind. None of them sail faster than the wind while sailing dead square which is sailing 180 deg to true wind ie with wind directly behind you.

There are 2 types of wind which are true wind and apparent wind.

If you are on a motorbike travelling at 60 KPH on a day with no wind you will have an apparent wind of 60 KPH in your face. If you have a 10 KPH tailwind then the apparent wind in your face would be 50KPH.

When sailing across the wind at 90 deg to true wind you could do a vector diagram at various wind and boatspeeds which will show apparent wind is coming from from 70 deg to maybe even 60 deg from bow if you're going faster.

If you do vector diagram at 135 deg from true wind with 10 knots windspeed and 15 knot boatspeed then the apparent wind is probably around 90 deg.

The fastest liquid water boats are capable of sailing at twice windspeed maybe a little more from around 80 deg to 135 deg from true wind direction which means apparent wind is always coming from forward of the beam.

The ICE yachts are the fastest sailing craft they can do around 4-5 times faster than the wind. They have 3 little ice skates which have very little drag.

The fastest boats never run square they always tack downwind. Keelboats which are usually limited by hullspeed (they don't plane) being 1.34 x Square root of waterline length in feet don't gain anything once hull speed is reached so they run square. There is something called VMG (velocity made good) which means some boats aren't fast enough to gain much by tacking downwind so they don't do it.

No sailboats sail faster than the wind with wind directly behind ie 180 deg to true wind we call that running square. With a fast sailboat pulling away to run square will slow the boat down.

Can you make sense of this?

https://www.quora.com/Can-a-sailboat-travel-faster-than-wind-to-the-direction-of-wind-using-beating-For-example-can-a-sail-boat-win-a-balloon-floating-downwind

Can a sailboat travel faster than wind to the direction of wind using beating? For example, can a sail boat win a balloon floating downwind?

Yes the sailboat will sail downwind faster than a balloon provided it use a spinnaker and jibe downwind. If you do it right the speed margin can be a significant amount.
If that was not thru no sailors would spend big money on a good spinnaker. We all use them and develop a skill set that work well downwind using one. After 50 years of racing sailboats and studying the aerodynamics related to sails, I pretty much got both the downwind and upwind working reasonably well.

I figured it out ... the propeller provides it's own thrust once it starts rotating. The power source for a propeller is usually an engine of some sort, but in this case it's the wind. The wind speed plus the thrust from the propeller equals a greater speed than the wind

Just because the wind is turning the propeller, doesn't mean the propeller is not doing it's job of pushing the vehicle along, it only needs a power source to make it rotate

The wheels turn the propeller.

freediver wrote on May 9^th, 2022 at 9:46pm:

The wheels turn the propeller.

The propeller turns the wheels

freediver wrote on May 9^th, 2022 at 9:33pm:

Can you make sense of this? https://www.quora.com/Can-a-sailboat-travel-faster-than-wind-to-the-direction-of-wind-using-beating-For-example-can-a-sail-boat-win-a-balloon-floating-downwind Can a sailboat travel faster than wind to the direction of wind using beating? For example, can a sail boat win a balloon floating downwind? Yes the sailboat will sail downwind faster than a balloon provided it use a spinnaker and jibe downwind. If you do it right the speed margin can be a significant amount. If that was not thru no sailors would spend big money on a good spinnaker. We all use them and develop a skill set that work well downwind using one. After 50 years of racing sailboats and studying the aerodynamics related to sails, I pretty much got both the downwind and upwind working reasonably well.

When they're talking about beating it means sailing upwind and yes some sailboats can sail faster than the wind while going upwind. The 18ft Skiffs i sailed could do 14 knots upwind in 10 knots of wind tacking through 90 deg which means they're sailing 45 deg away from true wind.

There are boats that don't have spinnakers that sail faster than the wind while tacking downwind.

The Moth is a small boat that can sail faster than the wind upwind, across the wind and downwind if there is enough wind for them to get up on hydrofoils and not too much wind so they can control it.

https://www.youtube.com/watch?v=KWB-hwan86s

I saw a huge debate on the video you posted about that concept around 10 years ago. The one doubt i have with your video is with that streamer on the bow flowing back was that caused by apparent wind coming from forward direction or suction from the prop?

The things that keep me awake at night:

How does a sailboat generate thrust while travelling downwind faster than the wind?


Quote:

The one doubt i have with your video is with that streamer on the bow flowing back was that caused by apparent wind coming from forward direction or suction from the prop?

Or wind shear - ie the wind speed is lower closer to the ground.

Blackbird is powered by the difference in speed between the ground and the air. If it were able to work in no wind, then it would be a perpetual motion machine, that is, violate conservation of energy.

The challenge of travelling downwind faster than the wind is essentially one of gearbox design (in which the whole vehicle is a gearbox), and achieving a effective coupling between the vehicle and the air.

Blackbird's propeller is coupled to the wheels. It is not a conventional vehicle powered by a wind turbine. It is an airplane powered by the ground. The ground drives the wheels, the wheels drive the propeller, and the propeller pushes the vehicle forward. If the propeller thrust force is greater than the braking force at the wheels, blackbird will accelerate and achieve greater than wind speed. 'Thrust' because this force acts to speed the vehicle up. 'Braking' force because this force acts to slow the vehicle down.

Consider the blackbird travelling at wind speed. This is equivalent to the treadmill prototype (shown in the video in my opening post) being held stationary on a treadmill, just with a different reference frame. Power is transferred to the vehicle through the wheels. This power is the braking force into the wheels multiplied by the relative speed between the vehicle and the ground. Thrust power is the thrust force from the propeller multiplied by the relative speed between the vehicle and the air. Because the air and the ground are travelling at different speeds, it is theoretically possible for the thrust force to be higher than the braking force, but the thrust power to be less than the braking power. Energy is conserved, and the difference in power is accounted for by all the inefficiencies. The trick here is to use the bulk air speed. The local air speed variation generated around the propeller is part of the 'coupling problem', one of the inefficiencies that limited blackbird to 2.8 times air speed.

I think the treadmill prototype is a better experiment, because it simplifies the analysis conceptually. The vehicle could be clamped to the treadmill frame so it does not move, and the question then becomes whether the thrust force can exceed the braking force. You are effectively turning the treadmill into a fan, and the interaction between the wheels and the tread is a cog in an elaborate gearbox. You can then adjust the gear ratio so that the force on the final gear (the propeller) is higher than on one of the intermediate gears (the wheels), with energy conserved by gearing them to turn at different speeds.

An even simpler analogy would be if you used a prototype fixed to a treadmill, and replaced the propeller with a screw (or threaded rod) mounted into a fixed plate. You could generate extremely high thrust forces because you do not have the coupling problem that a fan does.

Blackbird is a more complicated concept because all three parts are moving relative to each other: the ground, the wind, and the vehicle. A good analogy for this is a planetary gearbox in which all 3 shafts can turn, allowing the ring gear to turn. Using concentric shafts (one inside the other), such a gearbox can have two input shafts (analogous to the ground and air) turning at different speeds, and an output shaft (analogous to the vehicle frame) turning at a third speed. You can achieve any output speed you want with the right gear ratio.

Another good analogy is a 'car' mounted between two moving walls, with wheels coupled to both walls and a gearbox that makes the wheels turn at different speeds (and possibly opposite directions). Depending on the gear ratio and the relative speed of the two walls, the car could move in either direction, and at speeds far higher than either wall. In this case there is no limit to the car speed, because it does not have the coupling problem that blackbird does. You can bring this analogy one step closer to blackbird by thinking of one of the walls as a plate mounted under a blimp, allowing blackbird to get an almost 'rigid' coupling to the air.

freediver wrote on May 9^th, 2022 at 9:46pm:

The wheels turn the propeller.

Bias_2012 wrote on May 9^th, 2022 at 9:50pm:

The propeller turns the wheels

We are both right it turns out .... initially the wheels turn the propeller, but when the thrust of the propeller becomes greater than the wind, the propeller turns the wheels, and that's the only way the vehicle can go faster than the wind speed (if the propeller is designed right)


Go straight to 13mins and you'll see a model on a moving treadmill, which is the power source. The model doesn't get flung off the treadmill as you might expect, but instead starts traveling in the opposite direction due to the thrust of the propeller turning the drive wheels

Very counter intuitive


https://youtu.be/VUgajGv4Aok?t=795

In the vehicle's internal drive train, power is always going from the wheels to the propeller. If it is moving with the wind.

Some good explanations and diagrams of how modern sailboats can outrun a balloon.

https://sailing-blog.nauticed.org/americas-cup-apparent-wind/

Basically, if the boat is moving at an angle to the wind, when the vector component of your speed in the downwind direction matches the wind speed, you do not cease to get a propulsive force from the wind.

Assuming you are at 45 degrees to the wind direction, and moving with the wind (keeping pace with the balloon in the wind direction) the relative wind speed sideways across the boat is the same as when the boat is stationary. The relative wind speed parallel to the boat direction is also equal in magnitude, but has shifted from a tailwind to a headwind. The headwind produces drag, which a modern boat sees little of, and the sail uses the sideways wind component to generate a forward thrust. That thrust only has to be enough to overcome the drag for the boat to accelerate so that it's downwind speed becomes greater than that of the wind.

The boat moves in the direction the boat is pointing, because a good keel makes it move through the water like a strain stuck on a rail. A tight sail still presents a wall to the wind that appears to be stationary relative to the water, if you cannot see it's movement in the direction the boat is moving, and is thus still able to generate thrust from it.

Blackbird can also travel upwind. Wikipedia explains it pretty well.

A vehicle with a bladed rotor mechanically connected to the wheels can be designed to go at a speed faster than that of the wind, both directly into the wind and directly downwind. Upwind, the rotor works as a wind turbine driving the wheels. Downwind, it works as a propeller, driven by the wheels. In both cases, power comes from the difference in velocity between the air mass and the ground, as received by the vehicle's rotor or wheels.

https://en.wikipedia.org/wiki/Blackbird_(wind-powered_vehicle)

freediver wrote on May 11^th, 2022 at 7:33pm:

Some good explanations and diagrams of how modern sailboats can outrun a balloon. https://sailing-blog.nauticed.org/americas-cup-apparent-wind/ Basically, if the boat is moving at an angle to the wind, when the vector component of your speed in the downwind direction matches the wind speed, you do not cease to get a propulsive force from the wind. Assuming you are at 45 degrees to the wind direction, and moving with the wind (keeping pace with the balloon in the wind direction) the relative wind speed sideways across the boat is the same as when the boat is stationary. The relative wind speed parallel to the boat direction is also equal in magnitude, but has shifted from a tailwind to a headwind. The headwind produces drag, which a modern boat sees little of, and the sail uses the sideways wind component to generate a forward thrust. That thrust only has to be enough to overcome the drag for the boat to accelerate so that it's downwind speed becomes greater than that of the wind. The boat moves in the direction the boat is pointing, because a good keel makes it move through the water like a strain stuck on a rail. A tight sail still presents a wall to the wind that appears to be stationary relative to the water, if you cannot see it's movement in the direction the boat is moving, and is thus still able to generate thrust from it.

With sailing any course heading  to true wind direction greater than 90 deg (perpendicular to wind) is considered downwind.

When travelling directly downwind ie when course heading is parallel to wind direction we call that running square.

A sail is like an aeroplane wing it needs airflow over both sides to produce lift. When running square you don't have airflow over both sides of sail so sail or wing has stalled. When you come up into the wind from dead square say about 45 deg you get airflow over both sides of the sail so it works as intended like a wing.

A keel stops the boat sliding sideways (leeway) that's another topic.

When sailing downwind at 135 deg from true wind then the apparent wind will move towards the bow when the boat is moving at a good speed. The faster you go the further forward the apparent wind will move so in effect the fastest boats always sail with apparent wind coming from a forward direction which means technically they're always sailing upwind.

If you are sailing to windward (upwind) ie 45 degrees from true wind doing 20 knots in 20 knots of wind then you will have around 40 knots of apparent wind coming over the bow.

The first boats to exploit apparent wind sailing were the 18ft skiffs on Sydney harbour over 40 years ago. Take note of how they let mainsail out so it's perpendicular to boat centreline while hoisting spinnaker (running square) then they point boat up around 45 deg get on trapeze the boat takes off then sails are trimmed closer to centreline as apparent wind increases just like if boat was pointing much higher into wind.
https://www.youtube.com/watch?v=FPRRNfbcULA

With the 18 video you will notice sails flogging and going inside out because they're overpowered while going upwind.

The advantage of using a wing is they don't go inside out stuffing up lift-drag coefficient so more efficient. Hydrofoils reduce skin friction drag from water to near nothing along with wave impact drag. These Catamarans are the current state of the art.

Video has graphics showing wind direction with white streaks along with windspeed and boatspeed. Take note of how they never run square.
https://www.youtube.com/watch?v=oTK_bsofyh0

It's basic trig to work out even though they travel much further distance tacking downwind compared to running square they're going a lot faster so they get there quicker.

Older keel boats are limited by hull speed which is 1.34 x square root of waterline length in feet so  around 6.7 knots for a 25ft boat so people in those boats always sail shorter distance for better VMG ( velocity made good).

The origins of apparent wind sailing was developed on Sydney harbour and with sail gp video the skippers of the Japanese and USA boats are Aussies.